Fig. 1 (a) A plot of the normalized anti-Stokes power versus the phase-gain mismatch factor, Δk / g ₀ = (2k _p-k _a-k _s) / g ₀ (where g ₀ is the Raman gain coefficient times the peak power, which was assumed to be 10 kW), and L is the interaction length after Roos [5] and Nazarkin [9]. High efficiencies are predicted but require long interaction lengths. (b-d) Raman simulations with varying levels of complexity. (b) Raman conversion with FWM simulation including only pump (green), Stokes (red), and anti-Stokes (blue) fields. The effect of phase matching (SRS gain g ₀ = 10, Δk / g ₀ = 0.5 solid lines, Δk / g ₀ =0.05 dashed lines) can be seen by the increased threshold and conversion efficiency to anti-Stokes. (c) Simulation including cascaded SRS to the 2^nd Stokes field (black). (d) This simulation includes full FWM interaction between all four fields and shows rapid conversion into the 2^nd Stokes and pump fields.

Fig. 2 (a) A simplified schematic of the experimental setup. The capillary was strapped to an aluminum block with a V-groove, (b) shows perspective view and (c) an end on view, and placed entirely inside a windowed gas cell (not shown). Cameras were used to look into the cell to image the input (d) and output (e) faces of the capillary to aid in coupling the light into the capillary.

Fig. 3 The temporal profiles of Raman converted pulses for 80 psi of CO₂. (a) An experimental plot showing the various Raman converted pulses with an input pulse peak power of 1.2 MW. (b) Various experimental traces in ascending input peak power levels showing the threshold for Stokes conversion, the subsequent threshold for 2^nd Stokes conversion, and evidence of conversion to 3^rd Stokes. (c) Numerical simulation of conditions in (a) using Eq. (1). Note the scale change on the pump as compared to (a).

Fig. 4 (a) The phase-gain mismatch factor as a function of pressure for H₂ assuming a 10 kW peak power. Experimental spatial mode profiles are shown for 38 and 82 psi showing that higher order spatial modes can have lower thresholds than the fundamental as it becomes phase matched. (b) The conversion efficiencies for the various Raman modes using the four mode treatment of Eq. (1). A very narrow and therefore gain sensitive region of efficient anti-Stokes conversion is seen (solid blue) as compared to the three mode treatment (dashed blue).